Cryogenic cooling systems are often required to cool a sample. This allows the sample to be analysed using one or more forms of microscopy, such as Atomic Force Microscopy (AFM) or Scanning Probe Microscopy (SPM), by reducing atomic vibrations in the sample. This is achieved by use of a cooling device, such as a dilution refrigerator to cool a sample to a desired temperature and to maintain that temperature during the analysis or to slow any increase in temperature.
To avoid the need for the sample to be held within the cooling device and thereby requiring the sample to be placed within the cooling device while at ambient temperature, a sample is able to be held within a sample holder. Such sample holders, also known as “sample pucks”, are removably mountable to the cooling device. This allows the environment into which the sample holder containing the sample is to be placed, to be cooled before the sample holder is introduced into that environment and the sample holder to be exchanged while the system is cold.
Since direct analysis of a sample is usually required, the analysis equipment needs to be held within the sample holder as well as the sample. This means that everything required by the equipment during the analysis has to either be held within the sample holder or to be supplied from outside the sample holder during the analysis. For example, it is known to supply power to the equipment within the sample holder via cables.
To allow a sample holder to be removable, there are connectors on the sample holder that mate with connectors on the cooling device. This allows power, for example, to be provided to connectors on the cooling device that mate with corresponding connectors on the sample holder so as to power the analysis equipment on the sample holder.
While providing power and other electrical connections between a cooling device and sample holder is relatively straightforward, the ability to provide a connection that allows light to pass between the cooling device and sample holder is significantly more difficult. A suitable optical connection cannot easily be established since the removable nature of the sample holder needs to be maintained whilst also preventing attenuation of light.
When light needs to pass between the cooling device and sample, such as when an optical signal is produced by the analysis or light is needed to stimulate a response in the sample, optical fibres (also referred to herein as “fibres”) are commonly used. However, commercially available connectors for optical fibres are screw-fit connectors, which are incompatible with removably mountable sample holders. This is because there is no suitable access for connecting a screw-fit connector to a sample holder, especially if any increase in temperature is to be kept to a minimum when the sample holder is mounted to the cooling device.
The alternative to a screw-fit connector would be a push-fit connector. This is still unsuitable however, because for light to pass between the cooling device and sample holder, the fibres on each of cooling device and the sample holder have to be positioned such that the light is not attenuated when passing between the fibres. This is only achievable if the ends of the fibres through which light is transmitted in use are very close together, to maintain the signal strength, and ensure precise alignment with each other to avoid light emitted from one fibre not passing into the opposing fibre.
Due to temperature cycling, such connections are difficult to produce because the changes in temperature will affect the alignment and positioning of the fibre ends. This would make any such push-fit optical connection unreliable and cause the connection to deteriorate with use. Accordingly, a reliable optical connector is required that is able to withstand temperature cycling and repeated use so as to allow light to be passed between a cooling device and a sample holder.